Hybrid system for construction machine

ABSTRACT

When the actuator drive torque of a hydraulic pump is lower than a switching torque τ0, a controller closes a solenoid switching valve and makes a rotary electric machine function as a generator. With the increase in the actuator drive torque, the controller controls a pressure compensated flow control valve so that the flow rate of hydraulic fluid supplied from the hydraulic pump to a rotary hydraulic device decreases. When the actuator drive torque is higher than the switching torque, the controller opens the solenoid switching valve, makes the rotary electric machine function as an electric motor, and controls the pressure compensated flow control valve so that the hydraulic fluid discharged by the hydraulic pump is not supplied to the rotary hydraulic device. Consequently, shocks due to flow rate fluctuation are prevented when operational states of the rotary hydraulic device and the rotary electric machine are switched.

TECHNICAL FIELD

The present invention relates to a hybrid system for a construction machine in which a rotary electric machine having functions of both an electric motor and an electric generator is provided in addition to an engine.

BACKGROUND ART

An example of a hybrid system for a construction machine has been described in Patent Document 1, for example. In this system, a rotary electric machine having functions of both an electric motor and a generator is provided in addition to an engine. The rotary electric machine is directly connected with the output shaft of the engine so that the engine can drive both the rotary electric machine and a hydraulic pump. When the load on hydraulic actuators is light, the hydraulic actuators are driven by driving and rotating the hydraulic pump with the engine. In this state, the rotary electric machine is driven by surplus power of the engine and is made to function as a generator. Electric power generated by the generator is stored in a battery. When the load on the hydraulic actuators is heavy, the rotary electric machine is made to function as an electric motor, and the actuators are driven by driving the hydraulic pump with both the engine and the rotary electric machine.

Another example of the hybrid system has been described in Patent Document 2. In this system, the rotary electric machine is separated from the engine, and a rotary hydraulic device is connected to the rotary electric machine. The hydraulic pump and the rotary hydraulic device arranged in parallel are connected to the hydraulic actuators via a control valve. When the load on the hydraulic actuators is light, part of the hydraulic fluid discharged by the hydraulic pump is supplied to the rotary hydraulic device to make the rotary hydraulic device function as a hydraulic motor. The rotary electric machine is driven by the hydraulic motor (rotary hydraulic device) and is made to function as a generator. Electric power generated by the generator is stored in a battery. When the load on the hydraulic actuators is heavy, the rotary electric machine is made to function as an electric motor for driving the rotary hydraulic device, by which the rotary hydraulic device is made to function as a hydraulic pump. The hydraulic fluid discharged by the former hydraulic pump driven by the engine and the hydraulic fluid discharged by the rotary hydraulic device functioning as a hydraulic pump are both supplied to the hydraulic actuators.

PRIOR ART LITERATURE Patent Document

Patent Document 1: JPA 2001-173024

Patent Document 2: JPB 3875900

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In the hybrid system described in the Patent Document 1, the rotary electric machine as an electric component is formed integrally with the main body of the engine, and thus measures for protecting the rotary electric machine from the heat and vibration of the engine have to be taken. In contrast, in the hybrid system of the Patent Document 2 in which the rotary electric machine is separated from the engine, a higher degree of freedom of arrangement is achieved compared to the system of the Patent Document 1 and the rotary electric machine as an electric component can be isolated from the heat and vibration of the engine.

In the system of the Patent Document 2, however, the control valve is employed for switching the line of the hydraulic fluid discharged by the hydraulic pump driven by the engine and the line of the hydraulic fluid discharged by the rotary hydraulic device driven by the rotary electric machine. For the switching of the lines, the control valve includes two switching valves: a first switching valve for opening and closing the line of the hydraulic fluid discharged by the hydraulic pump driven by the engine and a second switching valve for opening and closing the line of the hydraulic fluid discharged by the rotary hydraulic device driven by the rotary electric machine. For the switching of the operational states of the rotary hydraulic device and the rotary electric machine (from the state in which the rotary hydraulic device functions as a hydraulic motor and the rotary electric machine functions as a generator to the state in which the rotary electric machine functions as an electric motor and the rotary hydraulic device functions as a hydraulic pump, or the switching in the reverse direction), the first switching valve and the second switching valve are switched in a manner like ON/OFF switching. Thus, when such switching of the operational states of the rotary hydraulic device and the rotary electric machine is carried out during the operation (work) of the construction machine, shocks occur due to sharp fluctuations in the flow rates of the hydraulic fluid flowing through the lines and cause trouble and difficulty in the operation of the construction machine.

It is therefore the object of the present invention to provide a hybrid system for a construction machine that makes it possible to arrange the rotary electric machine isolated from the heat and vibration of the engine while also enabling smooth switching of the operational states of the rotary hydraulic device and the rotary electric machine during the operation of the construction machine without causing any shocks due to the flow rate fluctuation.

Means for Solving the Problems

(1) In order to achieve the above object, the present invention provides a hybrid system for a construction machine, comprising: an engine; a main hydraulic pump which is driven by the engine; a plurality of actuators; a control valve which is connected to the main hydraulic pump via a first hydraulic fluid supply line for controlling the flow of hydraulic fluid supplied to the actuators; a rotary electric machine which has functions of both an electric motor and a generator; a rotary hydraulic device which is connected to the rotary electric machine, driven by the rotary electric machine and functions as a hydraulic pump when the rotary electric machine functions as an electric motor, and functions as a hydraulic motor and drives the rotary electric machine when the rotary electric machine functions as a generator; a second hydraulic fluid supply line which connects the rotary hydraulic device to the first hydraulic fluid supply line; an electricity storage device; and a bidirectional converter which is connected between the electricity storage device and the rotary electric machine for controlling the transfer of electric power between the electricity storage device and the rotary electric machine, wherein: the hybrid system comprises: a flow control valve device which is arranged in the first hydraulic fluid supply line and in the second hydraulic fluid supply line for controlling the ratio between the flow rate of the hydraulic fluid supplied to the control valve and the flow rate of the hydraulic fluid supplied to the rotary hydraulic device when the hydraulic fluid discharged by the main hydraulic pump is supplied to the rotary hydraulic device via the second hydraulic fluid supply line; a third hydraulic fluid supply line which connects the rotary hydraulic device to the first hydraulic fluid supply line downstream of the flow control valve device; a first switching valve which is provided in the third hydraulic fluid supply line to be capable of opening and closing; a drive torque detecting device which detects drive torque of the main hydraulic pump; and a control device which judges whether actuator drive torque for driving the actuators is higher than preset switching torque or not based on the drive torque of the main hydraulic pump detected by the drive torque detecting device, and when the actuator drive torque is equal to or lower than the switching torque, the control device switches the first switching valve to its closed position and controls the bidirectional converter so that the rotary electric machine functions as a generator, while also controlling the flow control valve device so that the flow rate of the hydraulic fluid supplied from the main hydraulic pump to the rotary hydraulic device decreases with the increase in the actuator drive torque, and when the actuator drive torque is higher than the switching torque, the control device switches the first switching valve to its open position, makes the rotary hydraulic device function as a hydraulic pump by controlling the bidirectional converter so that the rotary electric machine functions as an electric motor, and controls the flow control valve device so that the hydraulic fluid discharged by the main hydraulic pump is not supplied to the rotary hydraulic device.

In the hybrid system of the present invention configured as above, the need of integrally arranging the engine and the rotary electric machine is eliminated, the degree of freedom of arrangement of the rotary electric machine is increased, and the isolation of the rotary electric machine from the heat and vibration of the engine is made possible. Thanks to the improvement in the installation environment of the electric system, a hybrid system with higher reliability can be realized.

In the present invention, when the actuator drive torque is equal to or lower than the preset switching torque, the control device switches the first switching valve to its closed position and controls the bidirectional converter so that the rotary electric machine functions as a generator, while also controlling the flow control valve device so that the flow rate of the hydraulic fluid supplied from the main hydraulic pump to the rotary hydraulic device decreases with the increase in the actuator drive torque. When the actuator drive torque is higher than the switching torque, the control device switches the first switching valve to its open position, makes the rotary hydraulic device function as a hydraulic pump by controlling the bidirectional converter so that the rotary electric machine functions as an electric motor, and controls the flow control valve device so that the hydraulic fluid discharged by the main hydraulic pump is not supplied to the rotary hydraulic device. Therefore, the switching from the state in which the rotary hydraulic device functions as a hydraulic motor and the rotary electric machine functions as a generator to the state in which the rotary electric machine functions as an electric motor and the rotary hydraulic device functions as a hydraulic pump, or the reverse switching, can be conducted in a state in which the flow rate of the hydraulic fluid supplied from the main hydraulic pump to the rotary hydraulic device is substantially 0, and variable control of the flow rate is possible before or after the switching. Consequently, the operational states of the rotary hydraulic device and the rotary electric machine can be switched smoothly during the operation of the construction machine without causing any shocks due to the flow rate fluctuation.

(2) Preferably, in the above hybrid system (1), the flow control valve device includes: a throttle part which is provided in the second hydraulic fluid supply line; and a pressure compensated flow control valve which is arranged in the first hydraulic fluid supply line and in the second hydraulic fluid supply line, the pressure compensated flow control valve having a pressure compensation function of controlling the flow rate of the hydraulic fluid supplied to the rotary hydraulic device by controlling differential pressure across the throttle part when the hydraulic fluid discharged by the main hydraulic pump is supplied to the rotary hydraulic device via the second hydraulic fluid supply line, the pressure compensated flow control valve supplying the control valve with a flow of the hydraulic fluid corresponding to a demanded flow rate in priority to the throttle part when the control valve is operated, and when the actuator drive torque is equal to or lower than the switching torque, the control device controls the pressure compensated flow control valve so that the differential pressure across the throttle part and the flow rate of the hydraulic fluid supplied to the rotary hydraulic device decrease with the increase in the actuator drive torque.

With this configuration, the flow control valve device is capable of controlling the ratio between the flow rate of the hydraulic fluid supplied to the control valve and the flow rate of the hydraulic fluid supplied to the rotary hydraulic device when the hydraulic fluid discharged by the main hydraulic pump is supplied to the rotary hydraulic device via the second hydraulic fluid supply line. Further, the control device is capable of controlling the flow control valve device so that the flow rate of the hydraulic fluid supplied from the main hydraulic pump to the rotary hydraulic device decrease with the increase in the actuator drive torque when the actuator drive torque is equal to or lower than the switching torque.

(3) Preferably, in the above hybrid system (2), the pressure compensated flow control valve includes: a first pressure-receiving part for action in an opening direction, to which hydraulic pressure on the upstream side of the throttle part is lead via a first signal hydraulic fluid line; and a second pressure-receiving part for action in a throttling direction, to which hydraulic pressure on the downstream side of the throttle part is lead via a second signal hydraulic fluid line; when the rotary hydraulic device functions as a hydraulic motor, and the control device includes: a second switching valve which is arranged in the second signal hydraulic fluid line; and a controller which receives a signal inputted from the drive torque detecting device, and when the actuator drive torque is equal to or lower than the switching torque, the controller validates the pressure compensation function of the pressure compensated flow control valve by switching the second switching valve to a first position for leading the hydraulic pressure on the downstream side of the throttle part to the second pressure-receiving part, and when the actuator drive torque higher than the switching torque, the controller invalidates the pressure compensation function of the pressure compensated flow control valve by switching the second switching valve to a second position for connecting the second pressure-receiving part with a tank.

With this configuration, the control device is capable of controlling the flow control valve device so that the flow rate of the hydraulic fluid supplied from the main hydraulic pump to the rotary hydraulic device decrease with the increase in the actuator drive torque when the actuator drive torque is equal to or lower than the switching torque. When the actuator drive torque is higher than the switching torque, the control device is capable of controlling the flow control valve device so that the hydraulic fluid discharged by the main hydraulic pump is not supplied to the rotary hydraulic device.

(4) Preferably, in the above hybrid system (2), the pressure compensated flow control valve includes: a first pressure-receiving part for action in an opening direction, to which hydraulic pressure on the upstream side of the throttle part is lead via a first signal hydraulic fluid line; a second pressure-receiving part for action in a throttling direction, to which hydraulic pressure on the downstream side of the throttle part is lead via a second signal hydraulic fluid line; and a third pressure-receiving part which sets a target compensation differential pressure according to control pressure; when the rotary hydraulic device functions as a hydraulic motor, and the control device includes: a solenoid proportional pressure reducing valve which outputs the control pressure to the third pressure-receiving part; and a controller which receives a signal inputted from the drive torque detecting device, and the controller calculates target compensation differential pressure that decreases with the increase in the actuator drive torque and equals 0 when the actuator drive torque exceeds the switching torque, and controls the solenoid proportional pressure reducing valve so that the target compensation differential pressure is achieved.

With this configuration, the control device is capable of controlling the flow control valve device so that the flow rate of the hydraulic fluid supplied from the main hydraulic pump to the rotary hydraulic device decrease with the increase in the actuator drive torque when the actuator drive torque is equal to or lower than the switching torque, and so that the flow rate of the hydraulic fluid supplied from the main hydraulic pump to the rotary hydraulic device substantially equals 0 when the actuator drive torque has become equal to the switching torque.

(5) Preferably, in the above hybrid system (1), the control device determines a torque value by subtracting the generator torque of the rotary electric machine functioning as a generator from the bidirectional converter from the drive torque of the main hydraulic pump detected by the drive torque detecting device, and uses the torque value as the actuator drive torque.

With this configuration, the actuator drive torque can be determined without the need of providing the actuators with special sensors.

(6) Preferably, in the above hybrid systems (1)-(5), the drive torque detecting device is a torque sensor which is provided on a rotating shaft transmitting driving force of the engine to the main hydraulic pump.

With this configuration, the drive torque detecting means is capable of detecting the drive torque of the main hydraulic pump.

Effects of the Invention

According to the present invention, the need of integrally arranging the engine and the rotary electric machine is eliminated, the degree of freedom of arrangement of the rotary electric machine is increased, and the isolation of the rotary electric machine from the heat and vibration of the engine is made possible. Thanks to the improvement in the installation environment of the electric system, a hybrid system with higher reliability can be realized.

Further, according to the present invention, the switching from the state in which the rotary hydraulic device functions as a hydraulic motor and the rotary electric machine functions as a generator to the state in which the rotary electric machine functions as an electric motor and the rotary hydraulic device functions as a hydraulic pump, or the reverse switching, can be conducted in a state in which the flow rate of the hydraulic fluid supplied from the main hydraulic pump to the rotary hydraulic device is substantially 0, and variable control of the flow rate is possible before or after the switching. Therefore, the operational states of the rotary hydraulic device and the rotary electric machine can be switched smoothly during the operation of the construction machine without causing any shocks due to the flow rate fluctuation. Consequently, the construction machine is allowed to deliver excellent operating performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the device configuration of a hybrid system for a construction machine in accordance with an embodiment of the present invention.

FIG. 2 is a graph showing aperture area characteristics of first and second variable throttle parts of a pressure compensated flow control valve.

FIG. 3 is a schematic diagram showing the external appearance of a hydraulic shovel on which the hybrid system in accordance with this embodiment is mounted.

FIG. 4 is a flow chart showing processing functions of a controller.

FIG. 5 is a graph showing the relationship between subtraction torque values and target control pressure of a solenoid proportional pressure reducing valve (target compensation differential pressure) stored in a table in a memory of a controller.

FIG. 6 is a graph showing transition of main pump drive torque and generator drive torque caused by a change in actuator drive torque.

MODE FOR CARRYING OUT THE INVENTION

Referring now to the drawings, a description will be given in detail of a preferred embodiment in accordance with the present invention.

FIG. 1 is a schematic diagram showing the device configuration of a hybrid system for a construction machine in accordance with an embodiment of the present invention.

In FIG. 1, the hybrid system in accordance with this embodiment comprises an engine 1, a main hydraulic pump 3 which connected to the engine 1 via a rotating shaft 2, is driven by the engine 1, a plurality of actuators 5 a-5 g, a control valve 7 which connected to the hydraulic pump 3 via a first hydraulic fluid supply line 6, controls the flow of hydraulic fluid supplied from the hydraulic pump 3 to the actuators 5 a-5 g, a rotary electric machine 8 which is a component having functions of both an electric motor and an electric generator, and a rotary hydraulic device 9 which is connected with the rotary electric machine 8, and when the rotary electric machine 8 functions as an electric motor, the rotary hydraulic device 9 is driven by the rotary electric machine 8 and functions as a hydraulic pump (subsidiary pump), when the rotary electric machine 8 functions as a generator, the rotary hydraulic device 9 functions as a hydraulic motor and drives the rotary electric machine 8. The hybrid system further comprises a second hydraulic fluid supply line 11 which is connected the rotary hydraulic device 9 to the first hydraulic fluid supply line 6, a fixed throttle part 12 which provided the second hydraulic fluid supply line 11, a pressure compensated flow control valve 13 which is arranged in the first hydraulic fluid supply line 6 and in the second hydraulic fluid supply line 11. When the hydraulic fluid discharged by the main hydraulic pump 3 is supplied to the rotary hydraulic device via the second hydraulic fluid supply line 11, the pressure compensated flow control valve 13 controls the flow rate of the hydraulic fluid supplied to the rotary hydraulic device by controlling the differential pressure across the throttle part 12 (pressure compensation function). When the control valve 7 is operated, the pressure compensated flow control valve 13 supplies the control valve 7 with a flow of the hydraulic fluid corresponding to a demanded flow rate in priority to the throttle part 12. The hybrid system further comprises a solenoid proportional pressure reducing valve 14 which outputs a hydraulic signal for setting a target compensation differential pressure of the pressure compensated flow control valve 13, a solenoid switching valve 15 which switches the pressure compensation function of the pressure compensated flow control valve 13 between valid and invalid, a third hydraulic fluid supply line 16 which is connected the rotary hydraulic device 9 to the first hydraulic fluid supply line 6 downstream of the pressure compensated flow control valve 13 and a solenoid switching valve 17 which provided the third hydraulic fluid supply line 16.

The pressure compensated flow control valve 13 has a first variable throttle part 13 x for controlling the flow rate of the hydraulic fluid supplied to the control valve 7 via the first hydraulic fluid supply line 6 and a second variable throttle part 13 y for controlling the flow rate of the hydraulic fluid supplied to the rotary hydraulic device 9 via the second hydraulic fluid supply line 11.

FIG. 2 is a graph showing aperture area characteristics of the first and second variable throttle parts 13 x and 13 y of the pressure compensated flow control valve 13. The horizontal axis represents the spool stroke (0 when the spool is situated at the leftmost position in FIG. 1). In FIG. 2, the solid line represents the aperture area characteristic of the first variable throttle part 13 x and the chain line represents the aperture area characteristic of the second variable throttle part 13 y.

The aperture area of the first variable throttle part 13 x is at its maximum when the spool stroke equals 0 (when the spool is at the leftmost position in FIG. 1), decreases with the increase in the spool stroke (with the rightward movement of the spool from the leftmost position), and drops to 0 when the spool stroke reaches its maximum (when the spool reaches the rightmost position in FIG. 1). Conversely, the aperture area of the second variable throttle part 13 y equals 0 when the spool stroke equals 0 (when the spool is at the leftmost position in FIG. 1), increases with the increase in the spool stroke (with the rightward movement of the spool from the leftmost position), and reaches its maximum when the spool stroke reaches its maximum (when the spool reaches the rightmost position in FIG. 1).

Returning to FIG. 1, the pressure compensated flow control valve 13 also has a first pressure receiving part 13 a to which the hydraulic pressure on the upstream side of the throttle part 12 is lead via a first signal hydraulic fluid line 21 a, is moved in a direction for closing the second variable throttle part 13 y, a second pressure receiving part 13 b to which the hydraulic pressure on the downstream side of the throttle part 12 is lead via a second signal hydraulic fluid line 21 b, is moved in a direction for opening the second variable throttle part 13 y and a third pressure receiving part 13 c to which control pressure (hydraulic signal) outputted by the solenoid proportional pressure reducing valve 14 is lead, sets the target compensation differential pressure according to the control pressure. With this configuration, the pressure compensated flow control valve 13 has the pressure compensation function of controlling the flow rate of the hydraulic fluid supplied to the rotary hydraulic device 9 by controlling the differential pressure across the throttle part 12 when the hydraulic fluid discharged by the main hydraulic pump 3 is supplied to the rotary hydraulic device 9 via the second hydraulic fluid supply line 11, while supplying the control valve 7 with a flow of the hydraulic fluid corresponding to the demanded flow rate in priority to the throttle part 12 when the control valve 7 is operated.

In this system, the throttle part 12 and the pressure compensated flow control valve 13 form a flow control valve device which is arranged in the first hydraulic fluid supply line 6 and in the second hydraulic fluid supply line 11 for controlling the ratio between the flow rate of the hydraulic fluid supplied to the control valve 7 and the flow rate of the hydraulic fluid supplied to the rotary hydraulic device 9 when the hydraulic fluid discharged by the main hydraulic pump 3 is supplied to the rotary hydraulic device 9 via the second hydraulic fluid supply line 11.

The solenoid switching valve 15 is arranged in the second signal hydraulic fluid line 21 b. The solenoid switching valve 15 stays at a first position (leftward in

FIG. 1) when an electric control signal applied to its solenoid 15 a is OFF, and is switched to a second position (rightward in FIG. 1) when the control signal turns ON. At the first position leftward in FIG. 1, the solenoid switching valve 15 opens the second signal hydraulic fluid line 21 b, leads the hydraulic pressure on the downstream side of the fixed throttle part 12 to the second pressure receiving part 13 b, and thereby validates the pressure compensation function of the pressure compensated flow control valve 13. When switched to the second position rightward in FIG. 1, the solenoid switching valve 15 shuts off the second signal hydraulic fluid line 21 b, connects the second pressure receiving part 13 b with a tank, and thereby invalidates the pressure compensation function of the pressure compensated flow control valve 13. When its pressure compensation function is invalidated by the solenoid switching valve 15, the pressure compensated flow control valve 13 operates so as to supply all the hydraulic fluid discharged by the hydraulic pump 3 to the control valve 7.

In FIG. 1, the hybrid system in accordance with this embodiment further comprises joysticks 31 (only one joystick 31 is shown in FIG. 1 for convenience of illustration) for commanding the operations of the actuators 5 a-5 g, a key switch 32 for commanding the turning ON/OFF of the system power and the starting/stopping of the engine 1, a power mode switch 33 for commanding the setting of a power mode, a torque sensor 34 (drive torque detecting device) provided on the rotating shaft 2 (transmitting the driving force of the engine 1 to the hydraulic pump 3) for detecting the drive torque of the hydraulic pump 3, a controller 35, a battery 36 (electricity storage device), and a bidirectional converter 37 connected between the battery 36 and the rotary electric machine 8 for controlling the transfer of electric power between the battery 36 and the rotary electric machine 8.

Upon receiving an engine start instruction from the key switch 32, the controller 35 starts up the engine 1 by driving an unshown starter. The controller 35 also receives operational electric signals from the joysticks 31, executes a prescribed calculation process, and outputs command current signals to solenoid proportional valves in the control valve 7. The solenoid proportional valves in the control valve 7 are operated by the command current signal, by which corresponding main spools are switched and corresponding actuators are driven.

Further, the controller 35 receives an operation command signal from the power mode switch 33, a detection signal from the torque sensor 34 and a generator torque signal when the rotary electric machine 8 functions as a generator from the bidirectional converter 37, executes a prescribed calculation process, and outputs control signals to the solenoid proportional pressure reducing valve 14 and the solenoid 15 a of the solenoid switching valve 15.

In this system, the solenoid proportional pressure reducing valve 14, the solenoid switching valve 15 and the controller 35 form a control device having the following functions: The control device judges whether actuator drive torque (explained later) for driving the actuators 5 a-5 g is higher than preset switching torque τ0 (explained later) or not based on the drive torque of the hydraulic pump 3 detected by the torque sensor 34 (drive torque detecting device). When the actuator drive torque is equal to or lower than the switching torque τ0, the control device switches the solenoid switching valve 17 (first switching valve) to its closed position and controls the bidirectional converter 37 so that the rotary electric machine 8 functions as a generator, while also controlling the flow control valve device formed by the fixed throttle part 12 and the pressure compensated flow control valve 13 so that the flow rate of the hydraulic fluid supplied from the main hydraulic pump 3 to the rotary hydraulic device 9 decreases with the increase in the actuator drive torque. When the actuator drive torque is higher than the switching torque τ0, the control device switches the solenoid switching valve 17 (first switching valve) to its open position, makes the rotary hydraulic device 9 function as a hydraulic pump by controlling the bidirectional converter 37 so that the rotary electric machine 8 functions as an electric motor, and controls the flow control valve device formed by the fixed throttle part 12 and the pressure compensated flow control valve 13 so that the hydraulic fluid discharged by the main hydraulic pump 3 is not supplied to the rotary hydraulic device 9.

The construction machine on which the hybrid system in accordance with this embodiment is mounted can be a hydraulic shovel, for example. The plurality of actuators 5 a-5 g can be a swing hydraulic motor 5 a, traveling hydraulic motors 5 b, a boom hydraulic cylinder 5 c, an arm hydraulic cylinder 5 d, a bucket hydraulic cylinder 5 e, a swinging hydraulic cylinder 5 f and a blade hydraulic cylinder 5 g, for example.

FIG. 3 is a schematic diagram showing the external appearance of the hydraulic shovel.

The hydraulic shovel comprises a lower travel structure 101, an upper swing structure 102 mounted on the lower travel structure 101 to be rotatable, and a front work implement 104 connected to the front end of the upper swing structure 102 via a swing post 103 to be rotatable vertically and horizontally. The lower travel structure 101 is a traveling structure of the crawler type. An earth-removing blade 106 which is movable up and down is attached to the front of a track frame 105 of the lower travel structure 101. The upper swing structure 102 includes a swing stage 107 forming a base structure and a cabin (cab) 108 mounted on the swing stage 107. The front work implement 104 includes a boom 111, an arm 112 and a bucket 113. The proximal end of the boom 111 is connected to the swing post 103 with a pin. The distal end of the boom 111 is connected to the proximal end of the arm 112 with a pin. The distal end of the arm 112 is connected to the bucket 113 with a pin.

The upper swing structure 102 is driven and swung with respect to the lower travel structure 101 by the swing hydraulic motor 5 a (see FIG. 1). The swing post 103 and the front work implement 104 are driven and rotated (swung) right and left with respect to the swing stage 107 by the swinging hydraulic cylinder 5 f. The boom 111, the arm 112 and the bucket 113 are driven and rotated up and down by the expansion and contraction of the boom hydraulic cylinder 5 c, the arm hydraulic cylinder 5 d and the bucket hydraulic cylinder 5 e, respectively. Lower travel structure 101 are driven and rotated by right and left traveling hydraulic motors 5 b. The blade 106 is driven up and down by the blade hydraulic cylinder 5 g.

FIG. 4 is a flow chart showing the processing functions of the controller 35.

First, the controller 35 calculates a target control pressure (i.e., the target compensation differential pressure of the pressure compensated flow control valve 13) as the target value of the control pressure outputted by the solenoid proportional pressure reducing valve 14 and outputs a drive current corresponding to the calculated target control pressure to the solenoid proportional pressure reducing valve 14 (step S100). The solenoid proportional pressure reducing valve 14 operates according to the drive current and outputs control pressure corresponding to the target control pressure to the third pressure receiving part 13 c of the pressure compensated flow control valve 13. The third pressure receiving part 13 c of the pressure compensated flow control valve 13 sets the control pressure as the target compensation differential pressure as mentioned above.

The calculation of the target control pressure of the solenoid proportional pressure reducing valve 14 in the step S100 is executed as follows:

The controller 35 receives the detection signal inputted from the torque sensor 34 and determines the drive torque of the main hydraulic pump 3 (hereinafter referred to as “main pump drive torque”) from the detection signal. Further, the controller 35 receives control information on the rotary electric machine 8 from the bidirectional converter 37 and thereby calculates the drive torque of the rotary electric machine 8 functioning as a generator (hereinafter referred to as “generator drive torque”).

Subsequently, the controller 35 calculates the difference τp−τg between the main pump drive torque τp and the generator drive torque τg. Let “τa” represent the torque supplied to the actuators 5 a-5 g (hereinafter referred to as “actuator drive torque” as needed) as a part of the main pump drive torque τp, the aforementioned difference τp−τg equals τa. In other words, the controller 35 calculates the actuator drive torque τa by subtracting the generator drive torque from the main pump drive torque.

Subsequently, the controller 35 refers to a table stored in a memory by using the value of τp−τg (i.e., the actuator drive torque τa) and thereby calculates the target control pressure of the solenoid proportional pressure reducing valve 14 corresponding to the value.

FIG. 5 is a graph showing the relationship between the value of τp−τg and the target control pressure of the solenoid proportional pressure reducing valve 14 (target compensation differential pressure) stored in the table in the memory. In FIG. 5, the target control pressure of the solenoid proportional pressure reducing valve 14 is expressed as “Pc”. A relationship between the value of τp−τg and the target control pressure Pc of the solenoid proportional pressure reducing valve 14, like the one shown in FIG. 5, has been stored in the table in the memory. The relationship has been set so that the target control pressure Pc of the solenoid proportional pressure reducing valve 14 decrease with the increase in τp−τg and the target control pressure Pc equals 0 when τp−τg is over a preset threshold value (switching torque) τ0.

Here, the threshold value (switching torque) τ0 means allowable maximum torque (part of the output torque of the engine 1) that can be consumed by the main hydraulic pump 3. For example, considering factors such as the existence of an unshown pilot pump driven by the engine 1, mechanical loss, etc., the threshold value (switching torque) τ0 is set at a value obtained by subtracting torque corresponding to the factors from the maximum rated torque (output torque at the preset maximum revolution speed) of the engine 1.

The target control pressure Pc when τp−τg equals 0 is expressed as “Pc0”. Pc0 means the target control pressure when all the hydraulic fluid discharged by the main hydraulic pump 3 is supplied to the rotary hydraulic device 9 and the generator drive torque τg equals the main pump drive torque τp.

Subsequently, the controller 35 judges whether the power mode switch 33 is ON or not (step S110). If the power mode switch 33 is not ON, the controller 35 judges whether the value of τp−τg is higher than the preset threshold value (switching torque) τ0 (allowable maximum torque that can be consumed by the main hydraulic pump 3) or not (step S120). The value of τp−τg equals the actuator drive torque τa, and the threshold value (switching torque) τ0 equals the allowable maximum torque that can be consumed by the main hydraulic pump 3. Therefore, a case where the value of τp−τg is not higher than the preset threshold value (switching torque) τ0 means a case where the actuator drive torque τa is lower than the allowable maximum torque that can be consumed by the main hydraulic pump 3, that is, a case where the drive torque of the engine 1 still has a margin. Thus, in this case, in order to make the rotary hydraulic device 9 function as a hydraulic motor (i.e., in order to make the rotary electric machine 8 function as a generator), the controller 35 sets the drive current of the solenoid switching valve 15 to OFF (without drive current), sets the drive current of the solenoid switching valve 17 to OFF (without drive current), and controls the bidirectional converter 37 in a power generation/recharging mode (step S130).

When the power mode switch 33 is ON in the step S110 or difference between the main pump drive torque τp and generator drive torque τg is judged to be higher than the threshold value (switching torque) τ0 in the step S120, the drive torque of the engine 1 does not have a margin. In this case, in order to make the rotary hydraulic device 9 function as a hydraulic pump (i.e., in order to make the rotary electric machine 8 function as an electric motor), the controller 35 advances to step S140 and sets the drive current of the solenoid switching valve 15 to ON (with drive current), sets the drive current of the solenoid switching valve 17 to ON (with drive current), and controls the bidirectional converter 37 in a driving mode.

Next, the outline of the operation of this embodiment will be explained.

<Regular Operation>

The controller 35 constantly calculates the target control pressure Pc of the solenoid proportional pressure reducing valve 14 (target compensation differential pressure of the pressure compensating valve 13) and outputs corresponding drive current to the solenoid proportional pressure reducing valve 14 (step S100 in FIG. 4). According to the drive current supplied from the controller 35, the solenoid proportional pressure reducing valve 14 outputs control pressure that is equal to the target control pressure Pc. The pressure compensating valve 13 sets its target compensation differential pressure at a level equal to the target control pressure Pc.

<When Power Mode Switch 33 is OFF and τp−τg τ0>

When the power mode switch 33 is at its OFF position, the controller 35 monitors whether τp−τg (i.e., the actuator drive torque τa) is higher than the preset threshold value (switching torque) τ0 or not (step S120). If τp−τg is not higher than the preset threshold value (switching torque) τ0, the controller 35 sets the bidirectional converter 37, the solenoid switching valve 15 and the solenoid switching valve 17 in the following settings (step S130):

(1) solenoid switching valve 15→drive current: OFF

(2) bidirectional converter 37→power generation/recharging mode

(3) solenoid switching valve 17→drive current: OFF

By setting the drive current of the solenoid switching valve 15 to OFF, the pressure compensation function of the pressure compensated flow control valve 13 is validated, the differential pressure across the fixed throttle part 12 is controlled by the pressure compensated flow control valve 13, and the flow rate of the hydraulic fluid supplied from the main hydraulic pump 3 to the rotary hydraulic device 9 (flow rate through the fixed throttle part 12) is controlled according to the target compensation differential pressure of the pressure compensated flow control valve 13 (target control pressure Pc). Thus, as τp−τg (actuator drive torque τa) gradually decreases and approaches the threshold value (switching torque) τ0, the target compensation differential pressure of the pressure compensated flow control valve 13 (the differential pressure across the fixed throttle part 12) decreases and the flow rate of the hydraulic fluid supplied from the main hydraulic pump 3 to the rotary hydraulic device 9 (the flow rate through the fixed throttle part 12) also decreases. When τp−τg reaches τ0, the target compensation differential pressure (the differential pressure across the fixed throttle part 12) falls to 0 and the flow rate of the hydraulic fluid supplied from the main hydraulic pump 3 to the rotary hydraulic device 9 also falls to 0.

By setting the bidirectional converter 37 in the power generation mode, the rotary electric machine 8 is made to function as a generator. By setting the drive current of the solenoid switching valve 17 to OFF, the solenoid switching valve 17 is held at its closed position.

In this setting, part of the hydraulic fluid discharged by the main hydraulic pump 3 is supplied to the rotary hydraulic device 9 via the fixed throttle part 12 whose differential pressure is controlled by the pressure compensated flow control valve 13, and thus the rotary hydraulic device 9 rotates as a hydraulic motor. Consequently, the rotary electric machine 8 rotates passively and generates electric power.

That is, the rotary hydraulic device 9, the rotary electric machine 8 and the battery 36 are in the following states:

rotary hydraulic device 9: functioning as a hydraulic motor

rotary electric machine 8: functioning as a generator

battery 36: recharging state

In this state, the main hydraulic pump 3 is carrying out both the supplying of the hydraulic fluid to the actuators 5 a-5 g demanded by the control valve 7 (i.e., the driving of the actuators) and the recharging of the battery 36 at the same time.

Further, since the solenoid switching valve 17 is at the closed position, the hydraulic fluid to be supplied to the rotary hydraulic device 9 through the fixed throttle part 12 can be prevented from being supplied to the control valve via the third hydraulic fluid supply line 16, and the hydraulic fluid supplied to the control valve via the pressure compensated flow control valve 13 can be prevented from being supplied to the rotary hydraulic device 9 via the third hydraulic fluid supply line 16.

<When τp−τg>τ0>

When τp−τg (the actuator drive torque τa) is judged to be higher than the preset threshold value (switching torque) τ0 in the step S120, the controller 35 sets the bidirectional converter 37 and the solenoid switching valve 15 in the following settings (step S140):

(1) solenoid switching valve 15→drive current: ON

(2) bidirectional converter 37→driving mode

(3) solenoid switching valve 17→drive current: ON

By turning ON the drive current of the solenoid switching valve 15, the solenoid switching valve 15 is switched to its second position, the second pressure receiving part 13 b is connected with the tank, and the pressure compensation function of the pressure compensated flow control valve 13 is invalidated. In this case, the pressure on the upstream side of the fixed throttle part 12 acts on the first pressure receiving part 13 a of the pressure compensated flow control valve 13 via the first signal hydraulic fluid line 21 a in the direction for closing the second variable throttle part 13 y, and the pressure compensated flow control valve 13 operates so as to supply all the hydraulic fluid discharged by the hydraulic pump 3 to the control valve 7.

Further, by setting the bidirectional converter 37 to the driving mode, the rotary electric machine 8 functions as an electric motor. By turning ON the drive current of the solenoid switching valve 17, the solenoid switching valve 17 is switched to the open position.

In this setting, the rotary hydraulic device 9 is driven by the rotary electric machine 8 and functions as a hydraulic pump. Due to the switching of the solenoid switching valve 17 to the open position, the hydraulic fluid discharged by the rotary hydraulic device 9 flows through the third hydraulic fluid supply line, merges with the hydraulic fluid discharged by the main hydraulic pump 3, and is supplied to the control valve 7. In this setting, the rotary electric machine 8 rotates actively as an electric motor.

That is, the rotary hydraulic device 9, the rotary electric machine 8 and the battery 36 are in the following states:

rotary hydraulic device 9: functioning as a hydraulic pump

rotary electric machine 8: functioning as an electric motor

battery 36: discharging state

In this state, the hydraulic fluid discharged by the main hydraulic pump 3 driven by the engine 1 and the hydraulic fluid discharged by the rotary hydraulic device 9 (subsidiary pump) driven by the rotary electric machine 8 (electric motor) according to the electric power of the battery 36 are merged together and supplied to the control valve 7.

In the above operation, the torque (energy) necessary for driving the actuators 5 a-5 g is secured by the engine 1 and the rotary electric machine (electric motor) 8 (hybrid function).

An example of the operation of this embodiment will be explained below referring to FIG. 6.

FIG. 6 is a graph showing transition of the main pump drive torque τp and the generator drive torque τg caused by the change in the actuator drive torque τa. The reference characters (A)-(C2) in FIG. 6 represent data in cases where τa ≦τ0 (i.e., τp−τg≦τ0) and the rotary hydraulic device 9 functions as a hydraulic motor (i.e., the rotary electric machine 8 functions as a generator). The reference characters (D1)-(E2) represent data in cases where τa>τ0 (i.e., τp−τg >τ0) and the rotary hydraulic device 9 functions as a hydraulic pump (i.e., the rotary electric machine 8 functions as an electric motor).

(a) State A

In a state A, none of the joysticks 31 is being operated, none of the main spools of the control valve 7 is being operated, and none of the actuators 5 a-5 g is being driven. In this state, the actuator drive torque τa (torque supplied to the actuators 5 a-5 g) equals 0, all the hydraulic fluid discharged by the main hydraulic pump 3 is supplied to the rotary hydraulic device 9 via the throttle part 12, and the main pump drive torque τp equals the generator drive torque τg (τp=τg).

Since τp−τg equals 0, the target control pressure Pc is at the maximum Pc0 and the differential pressure across the throttle part 12 is controlled to be equal to Pc0 by the pressure compensated flow control valve 13.

In other words, when the battery 36 is low and needs recharging, the system is controlled to satisfy τp=τg=τ0 (state A). When the electric amount of the battery 36 is close to that at the completion of recharging, the system is controlled to satisfy τp=τg<τ0 since the generator drive torque τg decreases correspondingly.

(b) State A→B1→B2

When any one of the main spools of the control valve 7 is operated by operating one of the joysticks 31, part of the hydraulic fluid discharged by the main hydraulic pump 3 is supplied to a corresponding one of the actuators 5 a-5 g (hereinafter referred to as an “actuator 5 x” for convenience), by which the actuator 5 x is driven. In this case, a certain amount of actuator drive torque τa occurs and the main pump drive torque τp increases correspondingly. As a result, the main pump drive torque τp temporarily exceeds the preset threshold value (switching torque) τ0 (state B1).

When the main pump drive torque τp exceeds the threshold value (switching torque) τ0 as above, τp−τg increases to a positive value corresponding to τa, the target control pressure Pc decreases according to the increase in τp−τg, and the differential pressure across the fixed throttle part 12 controlled by the pressure compensated flow control valve 13 also decreases similarly.

Consequently, the flow rate of the hydraulic fluid supplied to the rotary hydraulic device 9 (flow rate through the fixed throttle part 12) decreases, the generator drive torque τg of the rotary electric machine 8 decreases, the main pump drive torque τp (which has increased temporarily) decreases similarly due to the decrease in the generator drive torque τg, and the system returns to a state in which τp=τ0 (state B2).

That is, the system is controlled to satisfy:

τp=τa+τg

and

τp=τ0

(c) State B2→C1→C2

When the actuator drive torque τa increases due to a factor such as an increase in the operation amount of the joystick 31 or an increase in the load pressure on the actuator, the main pump drive torque τp correspondingly increases again to temporarily exceed the threshold value (switching torque) τ0 (state C1). When the main pump drive torque τp exceeds the threshold value (switching torque) τ0, τp−τg increases further to a value corresponding to τa and the differential pressure across the fixed throttle part 12 controlled by the pressure compensated flow control valve 13 decreases further.

Consequently, the flow rate of the hydraulic fluid supplied to the rotary hydraulic device 9 (flow rate through the fixed throttle part 12) decreases, the generator drive torque τg of the rotary electric machine 8 decreases, the main pump drive torque τp (which has increased temporarily) decreases similarly due to the decrease in the generator drive torque τg, and the system returns to a state in which τp=τ0 (state C2).

That is, the system is controlled to satisfy:

τp=τa+τg

and

τp=τ0

(d) State C2→D1→D2

When the actuator drive torque τa increases further and exceeds the threshold value (switching torque) τ0, the main pump drive torque τp also increases correspondingly to exceed the threshold value (switching torque) τ0 (state D1) and τp−τg also increases further to a value corresponding to τa. Since τa exceed τ0 in this case, τp−τg also increases over τ0.

Consequently, in the process in which τp−τg approaches τ0 before exceeding τ0, the target control pressure Pc (target compensation differential pressure) gradually and continuously decreases toward 0, the differential pressure across the fixed throttle part 12 also decreases similarly, and the flow rate of the hydraulic fluid supplied from the main hydraulic pump 3 to the rotary hydraulic device 9 is controlled to decrease gradually. When τp−τg equals τ0, the target control pressure Pc equals 0 and the flow rate of the hydraulic fluid supplied from the main hydraulic pump 3 to the rotary hydraulic device 9 drops substantially to 0.

At the instant when τp−τg exceeds τ0, the processing by the controller 35 shifts from the step S130 to the step S140 in the flow chart of FIG. 4, by which the pressure compensation function of the pressure compensated flow control valve 13 is invalidated and the solenoid switching valve 17 is switched to the open position. Further, the bidirectional converter 37 is switched to the driving mode. Consequently, the rotary electric machine 8 functions as an electric motor and the rotary hydraulic device 9 functions as a hydraulic pump.

As above, before the switching from the state in which the rotary hydraulic device 9 functions as a hydraulic motor and the rotary electric machine 8 functions as a generator to the state in which the rotary electric machine 8 functions as an electric motor and the rotary hydraulic device 9 functions as a hydraulic pump, variable control is executed so that the flow rate of the hydraulic fluid supplied from the main hydraulic pump 3 to the rotary hydraulic device 9 decreases gradually and the flow rate substantially equals 0 at the point of the switching. As a result, the operational states of the rotary hydraulic device 9 and the rotary electric machine 8 can be switched smoothly during the operation of the construction machine without causing any shocks due to the flow rate fluctuation. Incidentally, also when the operational states of the rotary hydraulic device 9 and the rotary electric machine 8 are switched reversely, variable control is executed so that the flow rate substantially equals 0 at the point of the switching and increases gradually after the switching. Thus, also in the reverse switching, the operational states of the rotary hydraulic device 9 and the rotary electric machine 8 can be switched smoothly during the operation of the construction machine without causing any shocks due to the flow rate fluctuation.

After τp−τg has exceeded τ0, the controller 35 controls the drive torque of the rotary electric machine 8 according to the difference between the main pump drive torque τp and the switching torque τ0. The hydraulic fluid discharged by the main hydraulic pump 3 and the hydraulic fluid discharged by the rotary hydraulic device 9 (subsidiary pump) driven by the rotary electric machine 8 (electric motor) are merged together and supplied to the control valve 7 (hybrid function). Consequently, the main pump drive torque τp which has temporarily increased decreases and the system returns to a state in which τp=τ0 (state D2).

That is, let “τs” represent drive torque (subsidiary pump torque) when the rotary hydraulic device 9 functions as a hydraulic pump (subsidiary pump), the system is controlled to satisfy:

τs=τa−τ0

and

τp=τ0

(e) State D2→E1→E2

Also when the actuator drive torque τa exceeding the threshold value (switching torque) τ0 increases further, the main pump drive torque τp increases temporarily (state E1). Also in this case, the controller 35 controls the drive torque of the rotary electric machine 8 according to the difference between the main pump drive torque τp and the switching torque τ0, and the hydraulic fluid discharged by the main hydraulic pump 3 and the hydraulic fluid discharged by the rotary hydraulic device 9 (subsidiary pump) are merged together and supplied to the control valve 7 (hybrid function). Therefore, the main pump drive torque τp which has temporarily increased decreases and the system returns to a state in which τp=τ0 (state E2).

That is, the system is controlled to satisfy:

τs=τa−τ0

and

τp=τ0

In this embodiment configured as above, a main drive system is formed by the engine 1 and the main hydraulic pump 3, and a subsidiary drive system is formed by the rotary electric machine 8 and the rotary hydraulic device 9. Each drive system is formed integrally. However, the main drive system and the subsidiary drive system are not required to be integral with each other but are just connected together by hydraulic lines (the first through third hydraulic fluid supply lines 6, 11 and 16). Therefore, each drive system can be arranged with a high degree of freedom and the rotary electric machine 8 as an electric component can be isolated from the heat and vibration of the engine 1. Thanks to the improvement in the installation environment of the electric system, a hybrid system with high reliability can be realized.

Further, the switching from the state in which the rotary hydraulic device 9 functions as a hydraulic motor and the rotary electric machine 8 functions as a generator to the state in which the rotary electric machine 8 functions as an electric motor and the rotary hydraulic device 9 functions as a hydraulic pump, or the reverse switching, can be conducted in a state in which the flow rate of the hydraulic fluid supplied from the main hydraulic pump 3 to the rotary hydraulic device 9 is substantially 0, and variable control of the flow rate is possible before or after the switching. Therefore, the operational states of the rotary hydraulic device 9 and the rotary electric machine 8 can be switched smoothly during the operation of the construction machine without causing any shocks due to the flow rate fluctuation. Consequently, the construction machine is allowed to deliver excellent operating performance.

A variety of modifications can be made to the above embodiment without departing from the spirit and scope of the present invention. For example, while a hydraulic shovel has been taken as an example of the construction machine in the above embodiment, it is also possible to apply the present invention to various other construction machines (hydraulic crane, wheel shovel, etc.) and achieve similar effects.

While the pressure compensated flow control valve 13 is formed by one valve in the above embodiment, the pressure compensated flow control valve 13 may also be separated into two valves: a valve arranged in the first hydraulic fluid supply line 6 and a valve arranged in the second hydraulic fluid supply line 11. It is also possible to implement the fixed throttle part 12 and the pressure compensated flow control valve 13 by one valve by equipping the pressure compensated flow control valve 13 with the function of the fixed throttle part 12.

While the pressure compensation function of the pressure compensated flow control valve 13 is switched between valid and invalid by switching the solenoid switching valve 15 in the above embodiment, the switching of the pressure compensation function between valid and invalid may also be implemented by expansion and contraction of a piston device provided for biasing the second pressure receiving part 13 b of the pressure compensated flow control valve 13.

While the drive torque detecting device in the above embodiment is implemented by the torque sensor 34 arranged on the rotating shaft 2 transmitting the driving force of the engine 1 to the hydraulic pump 3, the drive torque may be determined also by detecting the discharge pressure and the tilting angle (displacement) of the hydraulic pump 3 and multiplying the detected values together.

Description of Reference Numerals

1 Engine

2 Rotating Shaft

3 Main Hydraulic Pump

5 a-5 g Actuator

6 First Hydraulic Fluid Supply Line

7 Control Valve

8 Rotary Electric Machine

9 Rotary Hydraulic Device

11 Second Hydraulic Fluid Supply Line

12 Fixed Throttle Part

13 Pressure Compensated Flow Control Valve

13 a First Pressure Receiving Part

13 b Second Pressure Receiving Part

13 c Third Pressure Receiving Part

14 Solenoid Proportional Pressure Reducing Valve

15 Solenoid Switching Valve (second switching valve)

16 Third Hydraulic Fluid Supply Line

17 Solenoid Switching Valve (first switching valve)

21 a First Signal Hydraulic Fluid Line

21 b Second Signal Hydraulic Fluid Line

31 Joystick

32 Key Switch

33 Power Mode Switch

34 Torque Sensor (drive torque detecting device)

35 Controller

36 Battery (electricity storage device)

37 Bidirectional Converter 

1. A hybrid system for a construction machine, comprising: an engine; a main hydraulic pump which is driven by the engine; a plurality of actuators; a control valve which is connected to the main hydraulic pump via a first hydraulic fluid supply line for controlling the flow of hydraulic fluid supplied to the actuators; a rotary electric machine which has functions of both an electric motor and a generator; a rotary hydraulic device which is connected to the rotary electric machine, driven by the rotary electric machine and functions as a hydraulic pump when the rotary electric machine functions as an electric motor, and functions as a hydraulic motor and drives the rotary electric machine when the rotary electric machine functions as a generator; a second hydraulic fluid supply line which connects the rotary hydraulic device to the first hydraulic fluid supply line; an electricity storage device; and a bidirectional converter which is connected between the electricity storage device and the rotary electric machine for controlling the transfer of electric power between the electricity storage device and the rotary electric machine, wherein: the hybrid system comprises: a flow control valve device which is arranged in the first hydraulic fluid supply line and in the second hydraulic fluid supply line for controlling the ratio between the flow rate of the hydraulic fluid supplied to the control valve and the flow rate of the hydraulic fluid supplied to the rotary hydraulic device when the hydraulic fluid discharged by the main hydraulic pump is supplied to the rotary hydraulic device via the second hydraulic fluid supply line; a third hydraulic fluid supply line which connects the rotary hydraulic device to the first hydraulic fluid supply line downstream of the flow control valve device; a first switching valve which is provided in the third hydraulic fluid supply line to be capable of opening and closing; a drive torque detecting device which detects drive torque of the main hydraulic pump; and a control device which judges whether actuator drive torque for driving the actuators is higher than preset switching torque or not based on the drive torque of the main hydraulic pump detected by the drive torque detecting device, and when the actuator drive torque is equal to or lower than the switching torque, the control device switches the first switching valve to its closed position and controls the bidirectional converter so that the rotary electric machine functions as a generator, while also controlling the flow control valve device so that the flow rate of the hydraulic fluid supplied from the main hydraulic pump to the rotary hydraulic device decreases with the increase in the actuator drive torque, and when the actuator drive torque is higher than the switching torque, the control device switches the first switching valve to its open position, makes the rotary hydraulic device function as a hydraulic pump by controlling the bidirectional converter so that the rotary electric machine functions as an electric motor, and controls the flow control valve device so that the hydraulic fluid discharged by the main hydraulic pump is not supplied to the rotary hydraulic device.
 2. The hybrid system for a construction machine according to claim 1, wherein: the flow control valve device includes: a throttle part which is provided in the second hydraulic fluid supply line; and a pressure compensated flow control valve which is arranged in the first hydraulic fluid supply line and in the second hydraulic fluid supply line, the pressure compensated flow control valve having a pressure compensation function of controlling the flow rate of the hydraulic fluid supplied to the rotary hydraulic device by controlling differential pressure across the throttle part when the hydraulic fluid discharged by the main hydraulic pump is supplied to the rotary hydraulic device via the second hydraulic fluid supply line, the pressure compensated flow control valve supplying the control valve with a flow of the hydraulic fluid corresponding to a demanded flow rate in priority to the throttle part when the control valve is operated, and when the actuator drive torque is equal to or lower than the switching torque, the control device controls the pressure compensated flow control valve so that the differential pressure across the throttle part and the flow rate of the hydraulic fluid supplied to the rotary hydraulic device decrease with the increase in the actuator drive torque.
 3. The hybrid system for a construction machine according to claim 2, wherein: the pressure compensated flow control valve includes: a first pressure-receiving part for action in an opening direction, to which hydraulic pressure on the upstream side of the throttle part is lead via a first signal hydraulic fluid line; and a second pressure-receiving part for action in a throttling direction, to which hydraulic pressure on the downstream side of the throttle part is lead via a second signal hydraulic fluid line; when the rotary hydraulic device functions as a hydraulic motor, and the control device includes: a second switching valve which is arranged in the second signal hydraulic fluid line; and a controller which receives a signal inputted from the drive torque detecting device, and when the actuator drive torque is equal to or lower than the switching torque, the controller validates the pressure compensation function of the pressure compensated flow control valve by switching the second switching valve to a first position for leading the hydraulic pressure on the downstream side of the throttle part to the second pressure-receiving part, and when the actuator drive torque higher than the switching torque, the controller invalidates the pressure compensation function of the pressure compensated flow control valve by switching the second switching valve to a second position for connecting the second pressure-receiving part with a tank.
 4. The hybrid system for a construction machine according to claim 2, wherein: the pressure compensated flow control valve includes: a first pressure-receiving part for action in an opening direction, to which hydraulic pressure on the upstream side of the throttle part is lead via a first signal hydraulic fluid line; a second pressure-receiving part for action in a throttling direction, to which hydraulic pressure on the downstream side of the throttle part is lead via a second signal hydraulic fluid line; and a third pressure-receiving part which sets a target compensation differential pressure according to control pressure; when the rotary hydraulic device functions as a hydraulic motor, and the control device includes: a solenoid proportional pressure reducing valve which outputs the control pressure to the third pressure-receiving part; and a controller which receives a signal inputted from the drive torque detecting device, and the controller calculates target compensation differential pressure that decreases with the increase in the actuator drive torque and equals 0 when the actuator drive torque exceeds the switching torque, and controls the solenoid proportional pressure reducing valve so that the target compensation differential pressure is achieved.
 5. The hybrid system for a construction machine according to claim 1, wherein the control device determines a torque value by subtracting the generator torque of the rotary electric machine functioning as a generator from the bidirectional converter from the drive torque of the main hydraulic pump detected by the drive torque detecting device, and uses the torque value as the actuator drive torque.
 6. The hybrid system for a construction machine according to claim 1, wherein the drive torque detecting device is a torque sensor which is provided on a rotating shaft transmitting driving force of the engine to the main hydraulic pump.
 7. The hybrid system for a construction machine according to claim 2, wherein the drive torque detecting device is a torque sensor which is provided on a rotating shaft (2) transmitting driving force of the engine to the main hydraulic pump.
 8. The hybrid system for a construction machine according to claim 3, wherein the drive torque detecting device is a torque sensor which is provided on a rotating shaft transmitting driving force of the engine to the main hydraulic pump.
 9. The hybrid system for a construction machine according to claim 4, wherein the drive torque detecting device is a torque sensor which is provided on a rotating shaft transmitting driving force of the engine to the main hydraulic pump.
 10. The hybrid system for a construction machine according to claim 5, wherein the drive torque detecting device is a torque sensor which is provided on a rotating shaft transmitting driving force of the engine to the main hydraulic pump. 